Originally, known as pincers and used to handle hot coals, pliers are an ancient invention of hand tools that hold, grip, cut and bite objects. Pliers have two members, joined side by side on an axis and rotate relative to each other. The joint of a pliers allows each member to lever against the other and enhances the force at the working end while the handle section is moved. The joint can be a hinge joint, as used in standard pliers, or a pivot joint as in common scissors. The working end of the members can be generally short as in pliers or generally long as in scissors. The length of the handle depends on the amount of leverage needed to produce a force at the working end. The pliers handle is longer than the working end and the scissors handle is shorter than the working end. Pliers and other hinged tools are based on a triangular hinged system in which the apex is the hinge while the floor is open and the working end is attached to the apex.
As a hand grips an object the long fingers pull it to the center and/or the proximal part of the palm of the hand. Each long finger has three joints that allow a range of finger positions. The joints between the metacarpal bones of the hand and the proximal bones of the long fingers of the hand are called the metacarpal phalangeal (MP) joints. The proximal end of the MP joints lie at the horizontal creases in the palm. The joints between the proximal finger bone and second or middle finger bone of the long fingers are called the proximal interphalangeal (PIP) joints. The joints between the middle bones and end or distal finger bones are called the distal interphalangeal (DIP) joints.
When the hand is flat the extensor muscles of the forearm contract to extend the joints of the long fingers. When extensor muscles relax the hand changes from extension to the neutral or resting position. The muscular forces of extension and flexion of the forearm muscles are balanced and all the joints in the hand are partially flexed or bent. When the long fingers of the hand simultaneously flex to pull an object toward the palm the angle of each joint is related to anatomical and physiologic factors including the length of the individual finger bones and muscle contraction. Sequential joint flexion progressively closes the hand by decreasing the joint angles. When a fist is formed the long fingers flex and their fingertips align to touch the palm of the hand. If the fingertips touch the palm near the horizontal crease (distal part of the palm) then the angle formed at the PIP joints is smaller than the angle at the MP joints. However, if the fingertips touch the palm nearer to the wrist then the angle formed at the MP joints is smaller than the angle at the PIP joints. The significance of the angle of the long finger joint is related to whether the distal or middle part of the long fingers pulls an object. If the distal bones of the long fingers are pulling then the PIP joints have greater flexion and smaller angles. However, when the middle bones of the long fingers pull then the MP joints have smaller angles.
The hand adapts to the shape objects as it pulls them to the palm. Therefore, an object's shape determines which long finger bones and forearm flexor muscles that pull. For example, if the distal segments of the long fingers pull the flat side of an object all sections of the forearm's deep flexor muscle contract. When the middle bones of the long fingers pull the convex side of a flat object, all sections of the forearm's superficial flexor muscle contract. In both cases, the pull is symmetric across similar bones of the long fingers and one muscle group is used. However, if the object being gripped is round, like a cylinder, then similar segments of the long fingers do not pull. Furthermore, the muscle sections used to pull the bone segments of the fingers are asymmetric. For example, a cylinder is gripped with the distal segment of the index finger, the middle segments of the middle finger and the ring finger along and the distal segment of the small finger. The tendons of the middle sections of the contracting superficial forearm muscle pull the middle finger segments of the middle and ring fingers. Whereas, The tendons of the outside sections of the contracting deep forearm muscle pull the distal segments of the index and ring fingers. Thus, these asymmetric muscle groups pull non-similar tendons from both the superficial and deep flexor muscles of the forearm to pull the bones. Of note, the tendons pulling the middle segments of the middle and ring fingers are adjacent to the median nerve. Pulling these tendons provokes compression and pressure on the median nerve in the carpal tunnel (CT).
When viewing the palm of the flat hand from the wrist the thenar eminence lies above the hypothenar eminence. The difference increases when the thumb opposes the long fingers. When the thumb opposes the long fingers and an object, like a cylinder, is pulled toward the proximal part of the palm it first contacts the thenar eminence. Then the object tilts toward the hypothenar eminence as the ring finger and the small finger flex further to increase grip. The added grip moves tips of the ring finger and small finger closer to the palm and out of alignment with the ends of the index finger and middle finger. This can produce discomfort in the wrist as the flexor tendons of the ring and small finger move in the CT against the transverse carpal ligament (TCL) and median nerve. The discomfort is enhanced when the space in the CT is small or is compromised by repetitive wrist injury.
As discussed above, pliers are hand tools based on the triangular lever system and combine two members at an axis of rotation or hinge. The handle members of pliers are commonly convex or straight. Like a lever, one handle member can be fixed and the other moves or both handle can move. The fixed handle member can be considered held in place where it touches the thenar eminence and the hypothenar eminence at the proximal part of the palm of the hand. Long finger flexion advances moving handle member toward the fixed member to close the working end. However, both handle members can be moved toward each other from the hinge.
The working end of common pliers is usually held near the radial side of the hand and the free end of pliers' handles rests near the ulnar side of the hand. The palm holds the proximal handle and the long fingers hold the distal handle. The free end of pliers' handles is spread to open the working end. Actuating the working end of the common pliers involves reaching with the distal segment of the small finger and the distal segment of the ring finger on the ulnar side of the hand to pull the distal handle member. Next, the middle bones of the middle finger and the index finger of the long fingers of the hand advance to pull the distal handle member of the pliers. Simultaneously, the ring finger and small finger advance so their middle bones also pull the distal handle member of the pliers. This progression is related to the distance required for the long fingers to reach the distal handle member because of the hinge. The triangular hinged system forces the smallest and weakest sections of the forearm flexor muscles for small finger and ring finger to squeeze the pliers handle.
There are reasons that many people have hand and wrist problems from repetitive use of common pliers. The wide free end makes for longer reach and harder work for the ring finger and small finger. By design, common pliers have concave or straight handles. This causes the proximal member to press into the CT area of the palm of the hand and transmits pressure to the transverse carpal ligament (TCL) and the underlying median nerve. Joint and ligament stress is present at the MP joints when the long fingers of the hand reach off center for the moving pliers handle. This is because the MP joints have limited side motion and the long fingers are forced to deviate in the radial direction to reach and grasp the moving handle. Such stresses from the long fingers deviating at the MP joints can cause a problem. Furthermore, common pliers are sometimes clumsy to use and are not made for single-handed operation. It takes one hand to stabilize while the other spreads the handles apart adding time to tasks.